As a whiteboard near the entrance reminds the constant flow of visitors, it is 17 days until caucus, the mysterious process by which the voters of Iowa have the first stab at picking each party’s nominee to run for president in November.

Eight years ago, in shabby temporary offices much like this, a young upstart from the neighbouring state of Illinois garnered global attention by beating frontrunner Hillary Clinton, who was left in third place on caucus day. For Barack Obama, that was the beginning the long journey that would lead him to become the first black president of the United States.

Though wary of invoking Obama by name, the Sanders activists hoping to defeat an even stronger-looking Clinton in 2016 are proposing something equally shocking: catapulting a self-proclaimed democratic socialist into the White House.

Until recently, the notion that this 74-year-old independent senator from Vermont might take more than a handful of state delegates from the former secretary of state, let alone win the national Democratic primary and one day sit in the Oval Office, was the subject of ridicule among knowing Washington political commentators.

The surprise early state polling has provoked a flurry of activity from Clinton’s Brooklyn headquarters. A gaze that had been fixed on potential Republican opponents has swung back to focus on the challenger in Clinton’s own party.

“We always expected these to be tight races,” insists Clinton spokesman Brian Fallon. “I don’t think people took us at our word when we projected that back in April, but I think you all believe us now.”

Sanders policies that were largely ignored, such as his plans to introduce government-run health insurance for all, have prompted lengthy press briefings by Clinton staffers suddenly demanding more detail. A mixed record on gun control is now the subject of daily attacks and every Sanders advert is scrutinised for signs he is reneging on a promise not to return fire over Clinton’s Wall Street ties.

“Because Senator Sanders has proven himself to be a very viable candidate, there is natural scrutiny being applied to his policy proposals,” Fallon explains.

Interviews with a half dozen Sanders staffers as they engage with voters on the ground in Iowa reveal a very different, and more confident, mood.

“The Clinton people are acting like they were caught by surprise this week,” says Robert Becker, state director for Sanders. “The kitchen sink is being thrown at us. It screams desperation.” Becker says the “onslaught of attacks” are “mind boggling”.

There is more than a whiff of vindication among older figures like Becker – one of several veteran Sanders operatives who provide a gruff contrast to the starry-eyed young volunteers. He recalls the galvanising effect of seeing the campaign’s early popularity over the summer dismissed by leaders in the local Democratic party establishment.

“We were told that Sanders does not know how to organise, so we printed that out and put it up on a wall,” he says, before rattling off the statistics that prove otherwise.

Becker’s team now has 27 field offices across the state; 101 paid staff; 12,000 volunteers who have done at least some work for the campaign; and 2,200 precinct captains who are trained to help turn out the vote on caucus day. Nationally, Sanders has had more than 2 million donations, providing the funds to currently outspend Clinton in TV and radio advertising in the state.

When he was interviewed by the Guardian in December , Sanders said the 28,000 Iowans he had met in 15 trips to the state would also go a long way to compensate for a lack of media attention. Becker cautions that many such attendees may have just been curious.

“Just because someone comes to an event, we don’t assume they are going to vote for us,” he says.

Instead, the local team are focusing on the even more laborious process of going door-to-door in the final four- or five-day push known as GOTC, or “Get out the caucus”.

Up to four shifts a day of volunteers are fanning out across the state to knock on the doors of undecided voters, in the hope of identifying names to add to the GOTC list.

An afternoon spent with one such group of “foot soldiers” suggests support is indeed surging for Sanders, but Clinton remains popular too and a surprising number of voters have not made up their minds.

I love Bernie. He has a lot of good things to say and he’s passionate. Sometimes he comes across a little angry

Marissa, floating voter, West Des Moines

“I’m still trying to work it out,” says Marissa, a typical floating voter, from her house on a quiet suburban street in west Des Moines. “I love Bernie. He has a lot of good things to say and he’s passionate.

“Sometimes he comes across a little angry. That’s the one thing that bothers me about him. Otherwise I think he’s fantastic. I have a Bernie sign in my garage I’m about to put out.”

But before the canvassers can add this house to their list, though, Marissa adds: “It’s Hillary or Bernie. I don’t know. She’s so cold and he’s kind of angry, those are the two things, but I like them both a lot.”

“I haven’t decided yet,” agrees Marissa’s neighbour, Lisa. “My husband is a Bernie supporter but I still have to do some research on both of them. I have to pay attention to more than just the commercials.”

With so much to play for among undecided voters like this, every house, every bumper sticker and every yard sign is eagerly scrutinised.

“They have told us at the office to never skip a door because that might be the person who puts us over the edge,” says Britton Hansen, an 18-year-old political science student at nearby Drake University.

Though more people express enthusiasm for Sanders when asked directly by his likeable young canvassers, in street after street the silent battle of the campaign logos is evenly matched.

‘Just because I am a Republican doesn’t mean I am dead to him’

“Just because I am a Republican doesn’t mean I am dead to him,” says Joshua, who requests details of the Feelthebern.org website beneath a POW “Missing in Action” flag on the next street over. “We just need somebody other than Hillary. So many people are voting for her out of nothing but name recognition.”

State director Becker claims there are many more like Joshua in towns in the traditionally more conservative west of the state, such as Sioux City and Council Bluffs, where Sanders has been drawing big crowds.

But he warns that Clinton still has the resources to deploy a formidable ground game of her own, something there are few official figures to prove or disprove.

“We think they have twice what we have,” he claims, “but what we have is enthusiasm.”

Certainly the heightened passion that is evident among audiences when Sanders speaks at rallies is matched in his buzzy campaign headquarters. There is, for example, what amounts to an international brigade of foreign volunteers, led by a 22-year-old British student and including a French man and Greek woman.

“Someone said to me at a Thanksgiving dinner in Ames: ‘Why are you meddling?’,” says Simon Bracey-Lane from Wimbledon, who believes Sanders is a much more plausible revolutionary than the British Labour Party leader, Jeremy Corbyn. “I said it needs meddling with. I think America is a deeply troubled country.”

And as a reminder that it is not just students working for Sanders, 38-year-old Des Moines catering manager Tracy Murphy runs “Wine Wednesdays” to encourage women to come together for phone-banking marathons.

“If you want people to get involved you have to make it fun,” says the precinct captain, taking a quick glug before the next phone call.

Four months ago, when campaign offices first started popping up across the state they had a remarkably homespun appearance, with hand-drawn signs and colourful caricatures of the candidate. Now, despite the campaign raising more than $70m since, it is notable that the walls of its state headquarters are still strewn with colourful homemade posters and paintings.

Clinton supporters argue not all is as wholesome as it appears in the Bernie camp, accusing the campaign, for example, of misleading voters in Iowa by withholding details of how they will pay for radical new policies such as single-payer health insurance.

“Now is not the moment to plunge the country back into a debate over healthcare,” adds senior Clinton policy adviser Jake Sullivan. “Our task now is to defend the affordable care act from Republicans and to build on it.”

But such criticism is relished by those who claim Sanders is the genuine “working class hero”.

“Just because people are saying Hillary Clinton might lose, it does not give her the right to dictate when we release our plans,” is the terse response from Becker.

In truth the policy disagreements between the two campaigns remain polite compared with relentless bickering in the Republican primary, but the sight of Donald Trump and Ted Cruz emerging unscathed from recent squabbling has heightened the stakes for Democrats and prompted a fierce row over the question of who might be best placed to beat whichever emerges victorious.

The Sanders camp points to polls that show its candidate – not the presumed establishment frontrunner – is now faring better in hypothetical contests against Trump and Cruz.

In private, Clinton strategists suggest this is only because the Republicans have not yet begun to attack Sanders’ socialist record.

But if the unlikely firebrand from Vermont can blunt Clinton’s momentum in the Iowa caucus, all the standard rules of American politics may have to be rewritten, all over again.